This invention relates to apparatus for and a method of exchanging an O-ring interposed between an in-core housing located at the lower end of a guide tube of an in-core monitor and an in-core flange attached to the in-core housing of a boiling water reactor (BWR) at the time of exchanging the in-core monitor.
FIG. 1 shows a usual BWR 5, in which a platform 2 used for loading and unloading a fuel assembly is mounted on the uppermost floor 1 of the reactor and main and auxiliary winches, not shown, are mounted on the platform 2 which is movable on the floor 1 by wheels 3. A pressure vessel 6 is disposed in the reactor 5 surrounded by a concrete shield 4 and water 7 fills the reactor and the vessel 6 (hereinbelow called reactor water 7). A steam drier 8 and a steam separator 9 are located at the upper portion of the pressure vessel 6. An upper grid (or top guide) 10 and a core supporting plate (or lower grid) 11 are arranged below the steam separator 9 with a predetermined spacing therebetween. The steam drier 8 and the steam separator 9 can be removed when an in-core monitor 12 is to be exchanged, and then, members located below the upper grid 10 can be observed through the reactor water 7 by an operator standing on the platform 2.
The upper end of the in-core monitor 12 engages the grid 10, and the monitor 12 extends downwardly through a guide tube 13 suspended from the supporting plate 11 and through the bottom cover 14 of the pressure vessel 6 to reach an in-core flange 16 which is secured by bolts 22 to an in-core housing 15 connected to the lower end of the guide tube 13 as shown in FIG. 2. The structure below the bottom cover 14 of the pressure vessel 6 is generally called a pedestal 17 and within the pedestal is located a platform 18 for exchanging a control rod driving mechanism. A groove 20 for draining the drain in the pedestal is formed at the bottom 19 of the pedestal.
FIG. 2 shows in detail the housing 15 and the flange 16 of the conventional BWR, in which a metal O-ring 21 is interposed between the housing 15 and the flange 16. The lower end of the in-core monitor is received in coaxial openings 23 and 24 of the housing 15 and the flange 16, respectively, and the tapered valve member 26 of the monitor 12 rests on the corresponding tapered valve seat 25 in the opening 24 so as to tightly press the valve member 26 against the valve seat 25 by the pressure of the reactor water 7, thereby preventing the leakage of the reactor water 7 from the opening 24. Screw threads 27 are formed on the inside surface of the lower portion of the opening 24 and an adapter 28 is screwed thereinto.
The inspection and the repair of the conventional in-core monitor 12 have been performed by using spring reel means, not shown, located on the bottom floor 19 of the pedestal 17. Therefore, workers had to work for a long time within the pedestal for inspecting or repairing the monitor, and this work is very dangerous for workers because they are exposed to radiations. Furthermore, in order to exchange the O-ring 21, it was required to remove the in-core monitor and the flange 16, but the reactor water 7 is likely to leak from the opening 23 when the monitor 12 is removed, so that it was necessary to provide a packing made of an elastic material for the opening 23 to prevent the leakage of the reactor water 7.